1. Field of the Invention
This invention relates in general to transducer electrodes for filters or delay lines utilizing the surface wave principle on piezoelectric substrates utilizing transducers formed with longitudinally weighted interdigital electrode fingers and with wider feeder portions.
2. Description of the Prior Art
It has been known that metallized surfaces on piezoelectric substrates reduce the propagation velocity of elastic surface waves in filters or delay lines. For interdigital transducers for elastic surface waves with longitudinally weighted electrode fingers, for example, this effect causes part of the wave front in the electrode finger overlap zone to pass through a comparatively heavily metallized zone whereas other parts of the wave front outside the overlap zone pass through a zone where the metallization is relatively small. The wave components passing through the smaller metallized zone lead the wave components passing through the greater metallized zone resulting in undesired distortion of the wave front and, thus, causes discrepancies in the experimental frequency response relative to the desired frequency response of the transmission element.
So as to eliminate this leading edge effect, it has been known to compensate for the difference in metallization outside the finger electrode overlap zone to provide so-called dummy fingers. Such dummy fingers can be either insulated from the other electrode fingers or connected electrically with adjacent electrode fingers.
Such known dummy fingers have been the same width as the other electrode fingers, in other words about one quarter of the synchronous wave length of the surface wave. Thus, a disadvantage is that the reflections of the surface wave at the finger edges are added at the synchronous frequency impairing the amplitude and phase frequency response precisely in the middle of the transmission band. So as to eliminate these undesired reflections, one solution has been previously proposed to form the electrode fingers as so-called split-fingers. For this purpose, each electrode finger is formed as a double finger but this produces an undesirable result in that the narrower metallized zones are very difficult to produce since they are very delicate and also they have a higher ohmic resistance and, thus, increase the electrical energy losses.